Titanium sheet

ABSTRACT

Provided are a titanium sheet which combines strength with formability and a plate for plate heat exchangers which was obtained from the titanium sheet. The titanium sheet has a crystal grain structure which comprises an α phase, and is characterized by containing 0.020-0.150 mass % Fe, 0.020-0.150 mass % O, and 0.002-0.100 mass % C, with the remainder comprising titanium and unavoidable impurities. The sheet is further characterized in that the sum of the contents (mass %) of the Fe and the C is at least 0.80 times the content (mass %) of the O and the concentration of C in the grain boundaries is 1.0 mass % or higher.

TECHNICAL FIELD

The present invention relates to a titanium sheet having both high strength and satisfactory formability.

BACKGROUND ART

Titanium materials generally have high specific strength and excellent corrosion resistance. Because of these properties, the titanium materials are used in exterior trim parts of camera bodies and other optical equipment, and of household electrical appliances; materials for ornaments such as eye glasses and watches; components of consumer products such as kitchen equipment; components of transportation equipment such as motorcycles and automobiles; and components of heat exchangers in chemical, power-generating, food manufacturing, and other plants.

Among these uses, the titanium materials have been more widely used in the heat exchangers, in particular in plates of plate heat exchangers. These are demanded to have a larger surface area by press forming into a corrugated shape so as to have higher heat exchange efficiency that is necessary as a prescribed property. To meet the demand, titanium sheets for use in the heat exchangers, in particular in plates of plate heat exchangers, are demanded to have excellent formability so as to be corrugated more deeply.

Such titanium sheets heavily used in these uses are prescribed in Japanese Industrial Standard (JIS) H 4600 (enacted on Jul. 1, 1964). The titanium sheets prescribed herein are further classified into grades such as Grade 1, Grade 2, and Grade 3 typically by the amounts (contents) of impurities such as iron (Fe) and oxygen (O), and the strength. With an increasing grade, the titanium sheets show higher minimum strength. The titanium sheets are used in different uses depending on the Japanese Industrial Standard grade.

Titanium sheets having low iron and oxygen contents as in JIS Grade 1 have low strength, but high ductility. For this reason, pure titanium sheets of JIS Grade 1 have conventionally been used for components that require excellent formability.

Increasing demands have been recently made not only for better heat exchange efficiency, but also for higher strength and lighter weight in the field of heat exchangers. To meet these demands, titanium sheets having higher strength typically of JIS Grade 2 and Grade 3 are to be applied to the heat exchangers. The high-strength titanium sheets, however, have poor formability and therefore require still better formability.

Industrial pure titanium sheets prescribed in JIS are metal materials each mainly including alpha-phase grain microstructures including hexagonal crystal (hexagonal close-packed; HCP) structure.

It is known that forming of titanium and other metal materials generally requires plastic deformation by slip deformation and twining deformation, where the slip deformation is caused by movement of dislocations.

Of slip systems, primary slip system of the alpha phase of titanium is prismatic slip {10-10}<11-20>, and the slip systems further include basal slip {0001}<11-20> and pyramidal slip. In addition, a {11-22}<11-23> twin can act in deformation upon press forming. Titanium, however, includes a smaller amount of active slip systems and does not allow two or more slip systems to act easily, as compared with iron/steel materials having a body-centered cubic structure (BCC) and aluminum materials having a face-centered cubic structure (FCC). Titanium is therefore known to be less plastic deformation.

In contrast, known processes for allowing titanium materials to have higher strength include two processes, i.e., a process of mainly increasing the contents of impurity elements such as oxygen and iron in a titanium material so as to offer higher strength; and a process of performing grain refinement of a titanium material so as to offer higher strength.

Disadvantageously, however, the titanium materials, when allowed to have higher strength by these conventional processes, have significantly impaired formability.

Based on the characteristics of titanium, techniques for allowing titanium materials to have better formability are disclosed as follows.

Patent Literature 1 (PTL 1) proposes a method for producing a pure titanium sheet. The method employs a pure titanium material containing Fe, Ni, and Cr in such contents (in weight percent) as to meet a condition specified by a predetermined relational expression, and oxygen (O) in a content of 900 ppm or less, with the remainder consisting of Ti and inevitable impurities. According to the method, the pure titanium material is subjected sequentially to cold rolling and annealing (heat treatment) at 600° C. to 850° C. so as to allow the resulting pure titanium sheet to have an average grain size of 20 to 80 μm, and the pure titanium sheet is then subjected to acid wash with an aqueous nitric hydrofluoric acid solution meeting a condition as specified by a predetermined relational expression.

PTL 2 proposes a titanium sheet having excellent ductility. The titanium sheet has a chemical composition containing H, O, N, and Fe in contents as prescribed in JIS H 4600 Grade 1 or Grade 2, and carbon (C) in a content of 50 to 800 ppm, with the remainder consisting of titanium and inevitable impurities.

CITATION LIST Patent Literature

PTL 1: Japanese Patent No. 3228134

PTL 2: Japanese Unexamined Patent Application Publication (JP-A) No. 2002-317234

SUMMARY OF INVENTION Technical Problem

Disadvantageously, however, the titanium sheets proposed in PTL 1 and PTL 2, when having higher contents of impurity elements such as oxygen and iron or when undergoing grain refinement so as to have higher strength, have low ductility and thereby have significantly inferior formability.

The present invention has been made in consideration of the problems or disadvantages and has an object to provide a titanium sheet that has strength and formability both at satisfactory levels.

Solution to Problem

Specifically, the present inventors found that, although there is a limit in improvement of ductility in a titanium sheet by containing carbon and aluminum in combination according to conventional techniques, the titanium sheet can have better ductility by finely controlling the amounts of iron, oxygen, and carbon to be added. The present inventors have also found that the distribution of carbon at grain boundaries affects the ductility improving effect, and that the titanium sheet can have still better ductility by finely controlling the degree of segregation (concentrating) of carbon at grain boundaries.

The titanium sheet has increasing strength with an increasing carbon content. However, the titanium sheet can effectively enjoy better ductility by the presence of carbon when having a carbon content within a certain optimal range. The present inventors made further investigations finely and found that the optimal range depends also on the contents of iron and oxygen. In particular, oxygen makes the titanium sheet not only have significantly effectively higher strength, but also have inferior ductility. For this reason, the oxygen content is preferably minimized in order to allow carbon to exhibit its effects more efficiently. Independently, the present inventors had a finding regarding iron as follows. The titanium sheet, when containing carbon, can more effectively have a better balance between strength and formability with an increasing iron content.

In addition, the present inventors found that carbon allows the titanium sheet to have a better balance between strength and ductility with an increasing degree of segregation of carbon at grain boundaries in the grain microstructure of the titanium sheet even at an identical carbon content, where the degree of segregation indicates the position or distribution of carbon present in the microstructure.

The present inventors made intensive investigations based on the findings and have found that the controls of the contents of iron, oxygen, and carbon, and the ratio among them allow the titanium sheet to have a better balance between strength and formability; and that the control of the titanium sheet to have a high degree of segregation of carbon at grain boundaries allows the titanium sheet to have still better formability. The present invention has been made based on these findings.

The present invention provides, according to an embodiment, a titanium sheet having a grain microstructure of alpha phase. The titanium sheet contains iron (Fe) in a content of 0.020 to 0.150 mass percent, oxygen (O) in a content of 0.020 to 0.150 mass percent, and carbon (C) in a content of 0.002 to 0.100 mass percent, with the remainder consisting of titanium and inevitable impurities. The total contents (in mass percent) of iron and carbon is equal to or more than 0.80 time the content (in mass percent) of oxygen. A carbon concentration at grain boundaries is 1.0 mass percent or more.

The titanium sheet, as having the configuration, has controlled contents of iron, oxygen, and carbon and a controlled ratio among them so as to allow two or more slip systems/twin systems to act, and has a better balance between strength and formability. The titanium sheet, as having a carbon concentration at grain boundaries of 1.0 mass percent or more, has still better formability.

The titanium sheet according to the present invention preferably has an average grain size of 5 to 80 μm.

The titanium sheet, when having such configuration, may more readily undergo dislocation slip deformation and/or twinning deformation upon forming while having strength held at certain level. This titanium sheet may therefore have still better formability.

The titanium sheet according to the present invention is usable in a plate heat exchanger.

The plate heat exchanger, when using the titanium sheet according to the present invention, can act as a plate heat exchanger having high strength and satisfactory formability.

Advantageous Effects of Invention

The titanium sheet according to the present invention, as having a predetermined chemical composition and a specific carbon concentration at grain boundaries, has strength and formability both at satisfactory levels.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1( a) is a plan view of a forming tool used in formability evaluation of a titanium sheet according to the present invention; and FIG. 1( b) is a cross-sectional view of the forming tool along the line E-E.

DESCRIPTION OF EMBODIMENTS

Next, the chemical composition of the titanium sheet according to the present invention will be described in detail below.

Chemical Composition

The titanium sheet according to the present invention has an alpha-phase (HCP structure) grain microstructure and contains Fe in a content of 0.020 to 0.150 mass percent, O in a content of 0.020 to 0.150 mass percent, and C in a content of 0.002 to 0.100 mass percent, with the remainder consisting of titanium and inevitable impurities. The total of contents (in mass percent) of Fe and C is equal to or more than 0.80 time the content (in mass percent) of 0. In addition, the titanium sheet according to the present invention has a carbon concentration at grain boundaries of 1.0 mass percent or more.

Fe: 0.020 to 0.150 mass percent

Iron (Fe) is an important element that allows the titanium sheet to have higher strength and better formability.

The titanium sheet, if having an iron content less than 0.020 mass percent, may have insufficient strength. This may require a larger amount of strain to be introduced so as to allow the titanium sheet to have higher strength, resulting in lower formability of the titanium sheet. To prevent this, the iron content is controlled to 0.020 mass percent or more.

In contrast, the titanium sheet, if having an iron content greater than 0.150 mass percent, may be produced with inferior productivity because of higher degree of iron segregation in the ingot. In addition, this titanium sheet may undergo titanium grain refinement due to a larger amount of formed beta phase and may thereby have lower formability.

To prevent this, the iron content is controlled to 0.150 mass percent or less.

The iron content is preferably 0.100 mass percent or less.

The iron content is more preferably 0.080 mass percent or less.

O: 0.020 to 0.150 mass percent

Oxygen (O) is an element that allows the titanium sheet to have higher strength, but causes the titanium sheet to have inferior formability.

The titanium sheet, if having an oxygen content less than 0.020 mass percent, may have lower strength. This may require a larger amount of strain to be introduced so as to allow the titanium sheet to have higher strength, resulting in lower formability of the titanium sheet. To prevent this, the oxygen content is controlled to 0.020 mass percent or more.

In contrast, the titanium sheet, if having an oxygen content greater than 0.150 mass percent, may become brittle and have lower formability. In addition, this titanium sheet may be susceptible to cracking or breakage and may have lower productivity.

To prevent these, the oxygen content is controlled to 0.150 mass percent or less.

The oxygen content is preferably 0.125 mass percent or less.

The oxygen content is more preferably 0.100 mass percent or less.

C: 0.002 to 0.100 mass percent

Carbon (C) is an element that allows the titanium sheet to have higher strength and better formability.

The titanium sheet, if having a carbon content less than 0.002 mass percent, may hardly have a carbon concentration at grain boundaries at the predetermined level, may fail to have an effectively better balance between strength and formability, and may have lower strength. To prevent this, the carbon content is controlled to 0.002 mass percent or more.

In contrast, the titanium sheet, if having a carbon content greater than 0.100 mass percent, may have strength higher than necessary and may have inferior formability.

To prevent this, the carbon content is controlled to 0.100 mass percent or less.

The carbon content is preferably 0.090 mass percent or less.

The carbon content is more preferably 0.080 mass percent or less.

Remainder

The “inevitable impurities” in the titanium sheet according to the present invention refers to impurity elements that are inevitably contained in an industrial pure titanium sheet. The impurity elements are representatively exemplified by nitrogen, hydrogen, chromium, and nickel. In addition, the inevitable impurities further include hydrogen and other elements that may be possibly contained in the product during production processes. The titanium sheet, if containing impurities in large contents, may hardly have strength and formability both at satisfactory levels. To prevent this, the titanium sheet is desirably one having contents of inevitable impurities as reduced as appropriate. The titanium sheet can have lower contents of inevitable impurities by using alloy raw materials containing impurities in low contents.

Compositional index R: 0.80 or more

The titanium sheet can have a better balance between strength and formability by controlling the correlation among the iron, oxygen, and carbon contents in addition to the control of these contents individually.

The total of iron and carbon contents (in mass percent) is controlled to equal to or more than 0.80 time the oxygen content (in mass percent).

The correlation among the iron, oxygen, and carbon contents can be expressed so that a compositional index R as specified by Formula (1) is 0.80 or more, where Formula (1) is expressed as follows, in which [Fe], [C], and [O] represent the contents (in mass percent) respectively of iron, oxygen, and carbon in the titanium sheet:

R=([Fe]+[C])/[O]  (1)

The compositional index R may be controlled by adding Fe, O, and C in appropriate amounts depending on the iron, oxygen, and carbon contents in titanium scrap to be used as a raw material to produce the titanium sheet, so as to control the iron, oxygen, and carbon contents in the titanium sheet. In this process, iron may be added typically in the form of iron powder; oxygen may be added typically in the form of titanium oxide; and carbon may be added typically in the form of titanium carbide TiC.

As described above, the titanium sheet has increasing strength with an increasing carbon content. In contrast, the titanium sheet may have effectively satisfactory ductility due to the presence of carbon in a certain optimal range of the carbon content. The optimal range of the carbon content may depend also on the iron and oxygen contents. In particular, oxygen significantly effectively contributes to higher strength of the titanium sheet, but also causes the titanium sheet to have inferior ductility. For these reasons, the oxygen content is preferably minimized so as to allow the titanium sheet to have more efficient effects of the presence of carbon. In addition, an increasing iron content is effective so as to allow the titanium sheet to more efficiently have a better balance between strength and formability due to the presence of carbon.

Accordingly, the compositional index R is controlled to 0.80 or more in terms of lower limit.

The titanium sheet, when having a compositional index R of 0.80 or more, can allow two or more slip systems/twinning systems to act and can have a better balance between strength and formability.

The compositional index R is preferably 0.85 or more.

The compositional index R is more preferably 0.90 or more.

The titanium sheet, if having a compositional index R less than 0.80, may fail to allow two or more slip systems/twinning systems to act and may have inferior formability.

The compositional index R is preferably 12.5 or less in terms of upper limit, within the ranges of the iron, oxygen, and carbon contents.

The titanium sheet, if having a compositional index R greater than 12.5, has a content of any of the elements iron, oxygen, and carbon out of the preferred range and may thereby have inferior balance between strength and formability as compared with a titanium sheet having a compositional index R of 12.5 or less.

The compositional index R is more preferably 10.0 or less.

The compositional index R is furthermore preferably 6.0 or less.

While remaining unknown, the detailed mechanism for these is surmised as follows. In the titanium sheet, oxygen and iron are dissolved in the titanium matrix. However, oxygen and iron are present in different forms even in an identical solute state, because oxygen is an interstitial element, but iron is a substitutional element. In addition, iron has a smaller solid solubility limit in the titanium sheet as compared with oxygen and causes the formation of beta phase at an iron content at certain level or more (about 0.05 mass percent or more). Based on these, oxygen and iron affect carbon probably in different ways.

Accordingly, the titanium sheet, as having iron, oxygen, and carbon contents meeting the condition specified by Formula (1), may have a better balance between strength and formability.

Carbon concentration at grain boundaries: 1.0 mass percent or more

The concentration of carbon at grain boundaries (segregation state of carbon at grain boundaries) may affect the ductility improvement effects of the titanium sheet. Fine control of the carbon concentration at grain boundaries (segregation of carbon at grain boundaries) may allow the titanium sheet to have better ductility. In addition, the fine control of the carbon concentration at grain boundaries more contributes to a balance between strength and formability of the titanium sheet, as compared with other solutions for higher strength (increase in content of oxygen, grain refinement, and impartment of prestrain).

The titanium sheet, if having a carbon concentration at grain boundaries less than 1.0 mass percent, may fail to have an effectively improved balance between strength and formability even when containing carbon in a predetermined content as a whole.

To prevent this, the carbon concentration at grain boundaries is controlled to 1.0 mass percent or more.

The carbon concentration at grain boundaries is preferably 2.0 mass percent or more.

The carbon concentration at grain boundaries is more preferably 5.0 mass percent or more.

The control of the carbon concentration at grain boundaries may be performed by an after-mentioned production method. Specifically, the control is performed by controlling a cold rolling reduction in a cold rolling process before final annealing. In addition, the control is also performed by controlling the annealing temperature and annealing time in the final annealing process.

The cold rolling process before final annealing, when performed at a low cold rolling reduction, may allow carbon to be more readily segregated (distributed) actively at grain boundaries. The final annealing process, when performed at a high annealing temperature, may allow carbon to be actively segregated at grain boundaries. The final annealing process, when performed for a long annealing time, may allow carbon to be actively segregated at grain boundaries.

Carbon acts as an interstitial element in the grain microstructure of the titanium sheet and is present as a solute when contained in a content within the range specified in the present invention. Regarding the position of carbon (carbon distribution), the titanium sheet has a better balance between strength and formability with an increasing degree of segregation (distribution concentration) of carbon at titanium grain boundaries, even when carbon is present in an identical carbon content in the titanium sheet as a whole.

While remaining unknown, the mechanism for this is surmised as follows. Twins and deformation structures are formed in the titanium sheet with proceeding of plastic deformation, and these cause the titanium sheet to undergo strain concentration at titanium grain boundaries, leading to fracture. Carbon, when segregated at the titanium grain boundaries, allows the titanium grain boundaries to have higher strength, and this may impede strain concentration at specific grain boundaries in the titanium sheet. This probably allows the titanium sheet to have a better balance between strength and formability.

Average grain size: 5 to 80 μm

The average grain size affects the formability of the titanium sheet. However, the titanium sheet according to the present invention, when having an average grain size within a common range (2 to 150 μm), can have advantageous effects of the present invention. Even when the average grain size falls within the common range, the titanium sheet, if having an average grain size less than 5 μm, may resist twinning deformation upon introduction of strain into the titanium sheet: and in contrast, the titanium sheet, if having an average grain size greater than 80 μm, may suffer from orange peel surfaces. Typically for these reasons, the titanium sheet in both cases may have somewhat low formability. To prevent this, the average grain size is preferably controlled to 5 to 80 μm. The titanium sheet, when having an average grain size in the range of 5 to 80 μm, may have better formability so as to have a higher formability index F as mentioned below, as compared with a titanium sheet having an average grain size out of the range.

The average grain size is more preferably 10 to 60 μm.

The control of the average grain size may be performed by the production method. Specifically, the control is performed by controlling the cold rolling reduction of the cold rolling before the final annealing process, and the annealing temperature and annealing time in the final annealing process.

The cold rolling before the final annealing process, when performed at a lower cold rolling reduction, may allow the titanium sheet to have a larger average grain size. The final annealing process, when performed at a higher annealing temperature, may allow the titanium sheet to have a larger average grain size.

However, the final annealing process, if performed at an excessively high annealing temperature and excessively approaching the beta transformation temperature (Tβ), may adversely affect the grain growth due to beta phase newly formed. The final annealing process, if performed for a longer annealing time, may cause the titanium sheet to have a larger average grain size.

The average grain size can be measured typically by orientation analysis of the microstructure by electron back scattered diffraction pattern (EBSD) analysis, where the microstructure is observed and analyzed by scanning electron microscopy (SEM). In EBSD, a sample is irradiated with an electron beam, and a crystal orientation is identified using an electron backscattered diffraction pattern (Kikuchi pattern) formed upon irradiation.

The average grain size may be determined in the following manner. A boundary having a misorientation of 5° or more in the SEM/EBSD measurement data is defined as a grain boundary. When a grain surrounded by the grain boundary is approximated to a circle having an identical area, the diameter of the circle is defined as an equivalent circle diameter of the grain. The equivalent circle diameters of grains in a number of 100 or more are averaged to give an average equivalent circle diameter. This measurement procedure is further performed at multiple points (e.g., five or more points) to give average equivalent circle diameters, and the average of the average equivalent circle diameters is defined as the average grain size.

Plate for Plate Heat Exchanger

The plate for plate heat exchanger according to the present invention is one obtained by working the titanium sheet according to the present invention into a predetermined shape such as a deeply corrugated shape by a known technique such as press forming.

The titanium sheet according to the present invention has the chemical composition and carbon distribution at grain boundaries as described above and thereby has strength and formability both at satisfactory levels. The titanium sheet according to the present invention therefore has excellent formability typically without cracking even when worked into a deeply corrugated shape upon working (forming) into a plate for plate heat exchanger. The plate for plate heat exchanger according to the present invention has satisfactory strength and can endure a severe use environment such as use in a heat exchanger for a long time.

Method for Producing Titanium Sheet

Next, the method for producing the titanium sheet according to the present invention will be described.

The titanium sheet according to the present invention can be produced by a conventional production method. The production method may include melting process and remelting process by consumable-electrode vacuum arc remelting (VAR), casting process, hot forging process, hot rolling process, process annealing process, cold rolling process, and final annealing process.

Ways to control the carbon concentration at grain boundaries (ways to induce carbon segregation) in the production processes of the titanium sheet according to the present invention are as follows.

Melting Process

Oxygen, iron, and carbon are added to a molten metal in the melting process. To uniformly disperse carbon in the resulting titanium sheet, carbon is preferably added to the molten metal not alone, but in the form of titanium carbide (TiC). This allows the titanium sheet to easily contain carbon even upon melting by VAR, which is a common mass production technique.

Cold Rolling Process

In the cold rolling process, cold rolling and annealing are repeated while selecting a rolling reduction and annealing conditions as appropriate according to cold rolling properties (e.g., edge cracking susceptibility and deformation load) of the material. Cold rolling immediately before the final annealing process may be performed at such a rolling reduction (e.g., a rolling reduction of 30% or more) as to ensure a sufficient amount of processing so that the material will be recrystallized in the final annealing process.

The cold rolling process before the final annealing is preferably performed at a cold rolling reduction of 85% or less. This condition restrains the growth of a recrystallization texture after the final annealing to give a smaller proportion of small angle grain boundaries and to give a larger proportion of large angle grain boundaries, where carbon is segregated with difficulty at the small angle grain boundaries, but easily segregated at the large angle grain boundaries.

The cold rolling reduction is preferably set lower and is more preferably 70% or less.

The cold rolling reduction is furthermore preferably 60% or less.

Final Annealing Process

In the final annealing process, carbon diffuses during annealing in a promoted manner and is thereby actively segregated at grain boundaries. The final annealing is preferably performed at a high temperature for a long time. The final annealing process will be described below separately in the case using a continuous annealing furnace and in the case using a batch annealing furnace (vacuum furnace).

Continuous Annealing Furnace

The final annealing using a continuous annealing furnace is preferably performed at an annealing temperature of 600° C. to 890° C.

The final annealing, if performed at an annealing temperature lower than 600° C., may fail to sufficiently cause the segregation of carbon at grain boundaries and may fail to allow the titanium sheet to have a carbon concentration at grain boundaries of 1.0 mass percent or more. The final annealing, if performed at an annealing temperature higher than 890° C., may cause significant grain growth subsequent to recrystallization occurred during the annealing, and a specific orientation is accumulated at a higher degree. This may increase the proportion of small angle grain boundaries at which carbon is hardly segregated, impede the segregation of carbon at grain boundaries contrarily, and cause the titanium sheet to hardly have a carbon concentration at grain boundaries of 1.0 mass percent or more.

The final annealing using the continuous annealing furnace is more preferably performed at an annealing temperature of 700° C. to 890° C.

Holding of the workpiece in the final annealing using the continuous annealing furnace is not essential (the workpiece may be held for zero minute), but the holding, when performed, is preferably performed for a holding time of 10 minutes or shorter.

The holding, if performed for a time longer than 10 minutes, may cause significant grain growth subsequent to recrystallization occurred during annealing, and a specific orientation may be present at a higher degree of accumulation (in a higher density). This may increase the proportion of small angle grain boundaries at which carbon is hardly segregated, impede the segregation of carbon at grain boundaries contrarily, and cause the titanium sheet to hardly have a carbon concentration at grain boundaries of 1.0 mass percent or more.

The holding in the final annealing using the continuous annealing furnace is more preferably performed for a holding time of 1 to 10 minutes.

Batch Annealing Furnace (Vacuum Furnace)

The final annealing using the batch annealing furnace (vacuum furnace) is preferably performed at an annealing temperature of 550° C. to 700° C.

The final annealing, if performed at an annealing temperature lower than 550° C., may fail to invite sufficient segregation of carbon at grain boundaries and may cause the titanium sheet to fail to have a carbon concentration at grain boundaries of 1.0 mass percent or more. The final annealing, if performed at an annealing temperature higher than 700° C., may cause significant grain growth subsequent to recrystallization occurred during annealing, and a specific orientation may be present at a higher degree of accumulation. This may increase the proportion of small angle grain boundaries at which carbon is hardly segregated, impede the segregation of carbon at grain boundaries contrarily, and cause the titanium sheet to hardly have a carbon concentration at grain boundaries of 1.0 mass percent or more.

The final annealing using the batch annealing furnace (vacuum furnace) is more preferably performed at an annealing temperature of 600° C. to 700° C.

Holding in the final annealing using the batch annealing furnace (vacuum furnace) is preferably performed for a time of 30 minutes to 4 hours.

The holding, if performed for a time of shorter than 30 minutes, may fail to cause sufficient segregation of carbon at grain boundaries and may cause the titanium sheet to fail to have a carbon concentration at grain boundaries of 1.0 mass percent or more. The holding, if performed for a time longer than 4 hours, may cause significant grain growth subsequent to recrystallization occurred during annealing, and a specific orientation may be present at a higher degree of accumulation. This may increase the proportion of small angle grain boundaries at which carbon is hardly segregated, impede the segregation of carbon at grain boundaries contrarily, and cause the titanium sheet to hardly have a carbon concentration at grain boundaries of 1.0 mass percent or more.

Holding in the final annealing using the batch annealing furnace (vacuum furnace) is more preferably performed for a time of 1 to 4 hours.

The titanium sheet after annealing, when bearing scale deposited on the surface, is preferably subjected to a descaling process such as salt bath heat treatment and/or acid wash treatment.

Examples

Hereinafter the present invention will be illustrated in a specific manner with reference to several examples by which the advantageous effects of the present invention have been verified, in comparison with comparative examples that do not meet one or more of the conditions specified in the present invention.

It should be noted, however, that the examples are by no means intended to limit the scope of the invention; that various changes and modifications can naturally be made therein without deviating from the spirit and scope of the invention as described herein; and all such changes and modifications should be considered to be within the scope of the invention.

Test Sample

Pure titanium ingots having iron and oxygen contents as given in Table 1 (according to JIS H 4600) were melted by VAR using the ingots as a consumable electrode to give molten metals. The molten metals were combined with a carbon source in the form of titanium carbide (TiC), cast so that the total of iron and oxygen contents (in mass percent) was equal to or more than 0.80 time the oxygen content (in mass percent) (the compositional index R was 0.80 or more) as indicated in Table 1, and yielded a series of titanium materials (titanium ingots) having a diameter of 400 mm and a length of 5000 mm and having an alpha-phase grain microstructure.

Next, the titanium materials were hot-forged at 1000° C. for 30 minutes, then hot-rolled at 800° C., and yielded a series of hot-rolled sheets having a thickness of 4.0 mm. After removing surface scale, the hot-rolled sheets were subjected to cold rolling and process annealing (at 750° C. for 5 minutes in a continuous annealing furnace). The workpieces were subjected to descaling by immersing in a salt bath in furnace and subjected to acid wash. In addition, the workpieces were subjected to cold rolling and final annealing under conditions given in Table 1, and yielded a series of test samples (Test Sample Nos. 1 to 27) each having a thickness of 0.5 mm. The final annealing was performed in a continuous annealing furnace or a batch annealing furnace (vacuum furnace). This allowed the test samples to have a carbon concentration at grain boundaries of 1.0 mass percent or more.

When the final annealing was performed as continuous annealing, the sample was immersed in a salt bath in furnace, descaled by acid wash, and cold rolling reductions before and after process annealing were adjusted so as to give a sheet thickness of 0.5 mm.

TABLE 1 Carbon Final concen- Alpha Strength cold tration phase 0.2% Formability Chemical rolling Final annealing at grain Average Yield Inden- Test composition (Notes) Compo- reduc- Holding Holding boundaries grain strength tation Formability sample (mass percent) sitional tion temperature time (mass size YS depth X index F number Fe O C index R (%) System (° C.) (hr) percent) (μm) (MPa) (mm) (mm) 1 0.029 0.044 0.038 1.523 75 Batch 660 4   4.6 48 206.8 4.5 0.182 2 0.066 0.077 0.015 1.052 70 Batch 570 1   1.2 16 226.7 4.2 0.042 3 0.036 0.038 0.086 3.211 60 Continuous 840 0.12 6.2 32 209.5 4.5 0.204 4 0.066 0.108 0.023 0.824 40 Continuous 870 0.10 3.4 21 230.1 4.3 0.169 5 0.048 0.063 0.051 1.571 65 Batch 620 4   4.3 37 218.4 4.4 0.175 6 0.129 0.071 0.035 2.310 30 Batch 600 3   2.8 24 239.5 4.2 0.144 7 0.087 0.114 0.006 0.816 65 Continuous 680 0.02 1.6 7 281.7 3.8 0.082 8 0.073 0.096 0.004 0.802 50 Continuous 690 0.05 1.9 29 282.2 3.8 0.086 9 0.058 0.129 0.093 1.171 55 Batch 685 4   11.5  41 332.6 3.6 0.289 10 0.066 0.041 0.067 3.244 70 Batch 650 3   8.7 22 212.1 4.5 0.225 11 0.083 0.141 0.031 0.809 60 Continuous 880 0.16 13.1  17 334.3 3.6 0.302 12 0.143 0.026 0.096 9.192 50 Continuous 850 0.05 1.2 3 212.1 4.3 0.025 13 0.045 0.021 0.072 5.571 70 Continuous 670 0.05 1.7 11 233.8 4.4 0.298 14 0.150 0.022 0.098 11.273 30 Batch 600 4   1.1 4 259.2 3.9 0.002 15 0.021 0.040 0.080 2.525 80 Continuous 640 0.16 1.5 4 294.1 3.7 0.081 16 0.021 0.040 0.080 2.525 40 Batch 700 4   16.2  82 200.4 4.4 0.031 17 0.066 0.077 0.001* 0.870 80 Batch 590 2    0* # 24 221.2 4.0 −0.202 18 0.077 0.128 0.153* 1.797 75 Batch 580 2    0.5* 22 297.3 3.5 −0.094 19 0.211* 0.048 0.028 4.979 80 Continuous 680 0.05  0* # 3 208.1 4.2 −0.107 20 0.095 0.161* 0.080 1.087 75 Continuous 690 0.02  0.4* 14 347.5 3.1 −0.092 21 0.061 0.143 0.043 0.727* 70 Batch 580 1    0.4* 4 359.6 3.0 −0.095 22 0.066 0.108 0.023 0.824  86* Batch 550 1    0.3* 17 234.6 3.9 −0.195 23 0.129 0.071 0.035 2.310 60 Continuous  570* 0.02  0.3* 14 279.1 3.6 −0.139 24 0.029 0.044 0.038 1.523 80 Continuous  895* 0.05  0.4* 72 183.2 4.4 −0.106 25 0.071 0.107 0.038 1.019 75 Batch 550  0.25*  0.4* 7 319.3 3.3 −0.118 26 0.036 0.038 0.042 2.053 50 Batch 700 5*    0.6* 48 206.2 4.3 −0.022 27 0.066 0.041 0.067 3.244 55 Continuous 860  0.30*  0.8* 59 218.6 4.2 −0.023 Notes: The remainder consists of Ti and inevitable impurities Data with an asterisk (*) are out of the scope of the present invention Data with a number sign (#) are lower than the minimum limit of detection

Evaluation of Carbon Concentration at Grain Boundaries

The evaluation of carbon concentration at grain boundaries was performed using a field emission transmission electron microscope (FE-TEM) and an energy dispersive X-ray spectrometer (EDX). Specifically, using JOEL JEM-2010F (FE-TEM) equipped with Noran Vantage (EDX), a test sample was inclined so that grain boundaries of the test sample were perpendicular to the observation direction, and a point analysis was performed on each grain boundary at an acceleration voltage of 200 kV and at 1000000-fold magnification while reducing the diameter of the electron beam to about 1 nm, and an EDX spectrum was measured. The electron beam was applied for a time of 30 seconds for the EDX spectrum measurement. Based on the spectrum, the carbon concentration at a grain boundary was analyzed. The carbon concentration at the grain boundary was analyzed at ten points in each field of view, and the average of measured carbon concentrations was calculated. The measurement was performed in five fields of view per test sample, the average of measured data was calculated, and was defined as the carbon concentration at grain boundaries.

Measurement of Average Grain Size of Alpha-Phase Grains

Microstructure observation was performed using an electron back scattered diffraction pattern (EBSD) detector (Oxford Instruments Nordlys II). The observation was performed on observation objects of the test sample each as an area of 0.5 mm in the rolling direction and 0.5 mm in the transverse direction (width direction) in the rolling plane in each of a surface layer portion, a portion one-fourth the thickness t, and a central portion all in the thickness direction of the test sample.

Boundaries having a misorientation of 5° or more were identified as grain boundaries in the microstructure observation. Based on the identified grain boundaries, equivalent circle diameters of individual grains were calculated. Based on the calculated equivalent circle diameters of hundred grains, an average equivalent circle diameter was calculated. This measurement was performed at any five points in each of the portions. In addition, the average of the average equivalent circle diameters at the any five points was calculated and defined as the average grain size.

Tensile Strength Evaluation

A No. 13 test specimen prescribed in JIS Z 2241 (enacted on Jul. 22, 1952) was sampled from each test sample in such a direction that the rolling direction of the test sample met the loading axis. Next, the test specimen was subjected to a tensile test according to JIS H 4600 at mom temperature so as to measure 0.2% yield strength (YS).

A test sample having 0.2% yield strength (YS) of the test specimen of 200 MPa or more was evaluated as accepted.

Formability Evaluation

The formability was evaluated by performing press forming, where the press forming simulated the forming into a plate (heat-exchanging unit) of a plate heat exchanger.

As illustrated in FIGS. 1( a) and 1(b), tools used herein had a forming portion of 100 mm by 100 mm. The forming portion had four ridges having a maximum height of 6.5 mm and being disposed at a pitch of 17 mm. The ridges each had a vertex having a shape with a radius R of curvature of 2.5. The ridges each had one bent portion in an intermediate portion, where the bent portion was bent in one direction. The ridges extend linearly from the bent portion toward both ends. The ridges were arranged in the forming portion from the intermediate bent portion to the both ends diagonally with respect to the edges of the forming portion, thus resembling a corrugated shape. A pressing machine used herein was a 80-ton pressing machine (Universal Plastic Working Machine supplied by AMINO Inc.).

The press forming was performed by the following procedure. Initially, each test sample was coated with a rust preventive oil (R303P). Next, each test sample was placed on the lower tool (die) so that the rolling direction of the test sample met the upward and downward direction (vertical direction) in FIG. 1( a), and the flange portion was held by a blank holder. The upper tool (punch) was then indented at a press forming rate of 1 mm per second.

The tool was indented into each test sample by 0.1 mm, and a maximum indentation depth X at which no cracking occurred in each test sample was determined.

A formability index F specified by Formula (2) was determined, and a test sample having a positive formability index F was evaluated as accepted. Evaluation results are indicated in Table 1. Formula (2) is expressed as follows:

F=X−(5.972−0.008×YS)  (2)

wherein X represents the indentation depth; and YS represents the 0.2% yield strength.

Examples

Test Sample Nos. 1 to 16 were titanium sheets that met all the conditions (chemical composition, compositional index R, and carbon concentration at grain boundaries) specified in the present invention and had excellent balance between strength and press formability.

Comparative Examples

Test Sample Nos. 17 to 27 failed to meet the conditions specified in the present invention, in particular failed to meet the condition for the carbon concentration at grain boundaries, and had poor balance between strength and press formability.

Test Sample Nos. 17 to 20 underwent low-level segregation of carbon at grain boundaries, had a carbon concentration at grain boundaries out of the specified range, and each had poor balance between strength and formability of the titanium sheet. In addition, Test Sample Nos. 18 to 20 had characteristics as follows.

Test Sample No. 18 had a carbon content greater than the range as specified in the present invention and had excessively high strength higher than necessary.

Test Sample No. 19 had an iron content greater than the range as specified in the present invention, thereby underwent the formation of a larger amount of beta phase, and underwent titanium grain refinement.

Test Sample No. 20 had an oxygen content greater than the range as specified in the present invention, had excessively high strength higher than necessary, and became brittle.

Test Sample No. 21 had a compositional index R lower than the range as specified in the present invention, underwent low-level segregation of carbon at grain boundaries, thereby had a carbon concentration at grain boundaries lower than the specified range, and thereby had poor formability due to its brittleness, although having high strength.

Test Sample No. 22 underwent final cold rolling performed at a high cold rolling reduction, underwent low-level segregation of carbon at grain boundaries, had a carbon concentration at grain boundaries lower than the specified range, and thereby had poor formability.

Test Sample No. 23 underwent final annealing performed at a low annealing temperature, underwent low-level segregation of carbon at grain boundaries, had a carbon concentration at grain boundaries lower than the specified range, and thereby had poor formability.

Test Sample No. 24 underwent final annealing performed at a high annealing temperature, underwent low-level segregation of carbon at grain boundaries, had a carbon concentration at grain boundaries lower than the specified range, and thereby had insufficient strength and poor formability.

Test Sample No. 25 underwent final annealing performed for a short time, thereby underwent insufficient annealing and low-level segregation of carbon at grain boundaries, had a carbon concentration at grain boundaries lower than the specified range, and thereby had poor formability.

Test Sample No. 26 underwent final annealing performed for a long time, underwent low-level segregation of carbon at grain boundaries, had a carbon concentration at grain boundaries lower than the specified range, and thereby had poor formability.

Test Sample No. 27 underwent final annealing performed for a long time, thus received excessive annealing, underwent low-level segregation of carbon at grain boundaries, had a carbon concentration at grain boundaries lower than the specified range, and thereby had poor formability.

While the present invention has been described in detail with reference to the specific embodiments and working examples thereof, it is to be understood that the invention be not limited by any of the details of description, but rather be construed broadly within its spirit and scope as set out in the appended claims. In addition, it is obvious that various changes, modifications, or equivalent arrangements may be made therein without departing from the spirit and scope of the invention.

REFERENCE SIGNS LIST

-   -   1 tool 

1. A titanium sheet comprising a grain microstructure of alpha phase, the titanium sheet comprising: iron (Fe) in a content of 0.020 to 0.150 mass percent; oxygen (O) in a content of 0.020 to 0.150 mass percent; carbon (C) in a content of 0.002 to 0.100 mass percent; and titanium and inevitable impurities, wherein: a total content (in mass percent) of the iron and the carbon is equal to or more than 0.80 times a content (in mass percent) of the oxygen; and a carbon concentration at grain boundaries is 1.0 mass percent or more.
 2. The titanium sheet according to claim 1, wherein the titanium sheet has an average grain size of 5 to 80 μm.
 3. The titanium sheet according to claim 1, wherein the titanium sheet is formed into a plate in a heat exchanger. 